Alkylation is a reaction in which an alkyl group is added to an organic molecule. Thus an isoparaffin can be reacted with an olefin to provide an isoparaffin of higher molecular weight. Industrially, the concept depends on the reaction of a C.sub.2 to C.sub.5 olefin with isobutane in the presence of an acidic catalyst producing a so-called alkylate. This alkylate is a valuable blending component in the manufacture of gasolines due not only to its high octane rating but also to its low aromatic and sulfur content.
Industrial alkylation processes have historically used concentrated hydrofluoric or sulfuric acid catalysts under relatively low temperature conditions. Acid strength is preferably maintained at 88 to 94 weight percent by the continuous addition of fresh acid and the continuous withdrawal of spent acid. As used herein, the term "concentrated hydrofluoric acid" refers to an essentially anhydrous liquid containing at least about 85 weight percent HF.
Hydrofluoric and sulfuric acid alkylation processes share inherent drawbacks including environmental and safety concerns, acid consumption, and sludge disposal. For a general discussion of sulfuric acid alkylation, see the series of three articles by L. F. Albright et al., "Alkylation of Isobutane with C.sub.4 Olefins", 27 Ind. Eng. Chem. Res., 381-397, (1988). For a survey of hydrofluoric acid catalyzed alkylation, see 1 Handbook of Petroleum Refining Processes 23-28 (R. A. Meyers, ed., 1986).
Hydrogen fluoride, or hydrofluoric acid (HF) is highly toxic and corrosive. However, it is used as a catalyst in isomerization, condensation, polymerization and hydrolysis reactions. The petroleum industry uses anhydrous hydrogen fluoride primarily as a liquid catalyst for alkylation of olefinic hydrocarbons to produce alkylate for increasing the octane number of gasoline. Years of experience in its manufacture and use have shown that HF can be handled safely, provided the hazards are recognized and precautions taken. Though many safety precautions are taken to prevent leaks, massive or catastrophic leaks are feared primarily because the anhydrous acid will fume on escape, possibly creating a vapor cloud that could be spread for some distance. Previous workers in this field approached this problem from the standpoint of containing or neutralizing the HF cloud after its release.
U.S. Pat. Nos. 4,938,935 and 4,985,220 to Audeh and Greco, as well as U.S. Pat. No. 4,938,936 to Yan teach various methods for containing and/or neutralizing HF acid clouds following accidental releases.
But it would be particularly desirable to provide an additive which decreases the cloud forming tendency of HF without compromising its activity as an isoparaffin-olefin alkylation catalyst. Solvents and complexing agents for hydrofluoric acid have, in the past, been disclosed for various purposes as noted in the following references.
U.S. Pat. No. 2,615,908 to McCaulay teaches thioether-HF-copper complex compounds and a method for preparing the same. Potential uses for the thioether-HF-copper composition compounds are listed from column 6, line 55 through column 8 at line 3. The method is said to be useful for purifying HF-containing vent gases from an industrial HF alkylation plant. See column 7, lines 10-24.
U.S. Pat. No. 3,531,546 to Hervert discloses a HF--CO.sub.2 catalyst composition which is said to be useful for alkylation as well as olefin isomerization.
U.S. Pat. No. 3,795,712 to Torck et al. relates to acid catalysts comprising a Lewis acid, a Bronsted acid, and a sulfone of the formula R--SO.sub.2 --R', where R and R' are each separately a monovalent radical containing from 1 to 8 carbon atoms or form together a divalent radical having from 3 to 12 carbon atoms.
U.S. Pat. No. 3,856,764 to Throckmorton et al. teaches an olefin polymerization catalyst comprising (1) at least one organoaluminum compound, (2) at least one nickel compound selected from the class consisting of nickel salts of carboxylic acids, organic complex compounds of nickel, or nickel tetracarbonyl and (3) at least one hydrogen fluoride complex prepared by complexing hydrogen fluoride with a member of the class consisting of ketones, ethers, esters, alcohols, nitriles, and water.
U.S. Pat. Nos. 4,025,577 and 4,099,924 to Siskin et al. report the use of alkylation catalyst compositions containing HF, a metal halide, and sulfolane.
U.S. Pat. No. 5,073,674 to Olah relates to hydrofluoric acid-containing alkylation catalysts which are said to exhibit decreased fuming tendency in comparison with heat HF.
Promoters such as alcohols, thiols, water, ethers, thioethers, sulfonic acids, and carboxylic acids are disclosed in combination with strong Bronsted acids such as HF, fluorosulfonic and trihalomethanesulfonic acids in U.S. Pat. No. 3,778,489 to Parker et al. The promoters are said to modify the activity of the strong Bronsted acids for alkylation.
Certain additives have recently been found to mitigate the cloud forming tendency of strong Bronsted acids useful in isoparaffin-olefin alkylation while preserving most of the acid's activity as an alkylation catalyst. Diluting the acid with such additives, while effective in improving safety, affects changes in process chemistry which complicate process control. Specifically, it has been found that diluting the acid with certain additives increases the sensitivity of the acid to conjunct polymer formation. The accumulation of these conjunct polymeric byproducts is believed to interfere with isoparaffin-olefin alkylation catalysis.